CONGRUENT MELTING OF BINARY COMPOUNDS WITH NONNEGLIGIBLE VAPOR-PRESSURE .1. THEORETICAL ASPECTS OF THE CONGRUENT MELTING OF A HYDRATE

A theoretical temperature-specific volume-molar fraction (T-v-x) phase diagram of the binary system A - H2O presenting a hydrate H with cong ruent melting is studied. The phase diagram is divided into two subsys tems by the one-component phase diagram of the hydrate H. The eutectic invariant planes which occur in both subsystems are described. The re lationships be tween the invariants are given. Isochoric and isopletha l theoretical sections are given. The one-component phase diagram of t he hydrate H is described. It is pointed out that the triple line soli d-liquid-vapour which relates the congruent melting is not necessarily perpendicular to the molar fraction axis. The means that the three ph ases, solid, liquid and vapour, have not necessarily the same composit ion. The T-v-x representation gives an explanation to the deviation be tween the maximum of the melting curve and the stoichiometric composit ion of the hydrate which is often observed in the T-x binary diagrams A - H2O.